The present invention relates to a method for controlling an inverter by means of space vector modulation, in particular for controlling an electric machine, wherein the inverter includes a plurality of controllable switches and is designed to provide a polyphase electric current, in particular in order to supply an electric machine with electric current in a polyphase form, wherein a reference phase angle is predefined, wherein the inverter is controlled in such a way that a plurality of different consecutive switching states of the switches is configured, in order to provide the electric current in the form of a current space vector.
The present invention furthermore relates to a device for controlling an inverter, in particular for controlling an electric machine, wherein the inverter includes a plurality of controllable switches which are connected for providing a polyphase electric current as a function of a reference phase angle, in particular in order to supply the electric machine with current in a polyphase form, including a control unit which is designed to control the inverter in such a way that the inverter assumes a plurality of consecutive different switching states of the switches, in order to provide the current in the form of a current space vector.
The present invention furthermore relates to a motor vehicle drive train including at least one electric machine for providing drive power, an inverter for controlling the electric machine, and including a device for controlling the inverter of the type described above.
Various control methods are known in the technical field of three-phase loads in general and electric three-phase machines in particular. Currently, the method of space vector modulation is generally preferred for controlling the three-phase load. In this control method, a space vector is formed by the consecutive setting of eight basic voltage vectors. In order to provide the phase voltage, the basic voltage vectors are switched in a pulse width-modulated manner, so that a corresponding control voltage is generated.
In the known control methods, the electrical loads are controlled by means of an inverter using power semiconductor switches. The setting of the eight consecutive basic voltage vectors for generating the voltage space vector is implemented by alternating the switching on and switching off of certain power semiconductor switches of the inverters. At very low rotational speeds of the space vector, or if the three-phase load is an electric machine, at low rotational speeds of the controlled electric machines, some of the power semiconductor switches are connected very frequently or for a very long time and are thus thermally loaded by an electric current flowing for a very long time or very frequently. Therefore, the power semiconductor switches must be designed for very long switch-on times and for very large currents, generally making the inverter technically complex.
In order to counteract an overload, in particular, a thermal overload, of the power semiconductor switches, it is, for example, provided in WO 2010/000548A2 to omit one of two switching states which switch to zero potential in certain pulse width modulation periods, in order to minimize the switching losses of the power semiconductor switches.
Since the load, in particular the thermal load, of individual power semiconductor switches of the inverter is a function of a phase angle of the provided current space vector, or some of the power semiconductor switches of the inverter are loaded differently for certain phase angles of the provided current space vector, it is, for example, provided in DE 10393516 T1 to use a certain zero vector in certain angle ranges of the provided current space vector in order to reduce the switching losses of the power semiconductor switches.
This has the disadvantage that in certain control situations, in particular at very low rotational speeds of the current space vector, some of the power semiconductor switches are permanently thermally loaded, and thus in certain situations, the inverter is loaded non-uniformly, and it is not possible to avoid overloading some of the power semiconductor switches.